1. Field of the Invention
This invention relates to carriers for electrostatic latent image development and to coating agents for use in such carriers. More particularly, this invention relates to carriers for electrostatic latent image development which are relatively unaffected by humidity and to coating agents, for use in such carriers, that cure rapidly upon heating and upon cure, have electrification properties which are relatively unaffected by humidity.
2. Description of the Related Art
In electrophotography and electrostatic printing, an electrostatic latent image is formed on a photosensitive material. Then, a two-component dry developer is brought into close proximity to the electrostatic latent image such that the toner in the dry developer becomes attached to the electrostatic latent image. Next, the toner is transferred to a transfer sheet, after which the electrostatic latent image is developed by affixing the toner to the transfer sheet and heating.
This two-component dry developer is comprised of a toner and a carrier for electrostatic latent image development. The toner is comprised of a thermosetting resin and a colorant. The carrier is typically iron powder or glass powder. The toner is held onto the surface of the carrier by frictional electrification i.e. electrical charge. The surfaces of the carrier arc coated with a film of a peel able coating, such as silicone resin for the purpose of preventing film formation (spending) by the toner.
The silicone coating agents that have been used conventionally for coating the surfaces of carriers have included, for example, silicone resin compositions comprised of silanol-containing organopolysiloxanes (see Japanese Patent Application Early Disclosure No. 56-106968 (1981), silicone resin compositions containing organopolysiloxanes, comprised of bifunctional siloxane units and trifunctional siloxane units, and organotin compounds (see Japanese Patent Application Early Disclosure No. 61-284775 (1986), and silicone resin compositions comprised of methylphenyl silicone resin in which the ratio of silicon atoms bonded organic groups to silicon atoms is i.5 and less (see Japanese Patent Application Early Disclosure No. 2-160259 (1990).
However, because these coating agents are cured by condensation reactions of the silanol groups, there has been the problem that cure does not occur sufficiently if they are not heated for a long time at a high temperature, for example 200.degree. to 250.degree. C. Further, because unreacted silanol groups remain locally in the cured film that is obtained by curing the coating agent, there have been problems with variations in the electrostatic properties of the carrier that is obtained depending upon curing conditions and that electrification properties vary greatly depending on humidity.
The inventors first proposed a coating agent for carriers for electrostatic latent image development comprised of organopolysiloxanes that were composed of bifunctional siloxane units having aminoalkyl groups and trifunctional siloxane units and organopolysiloxanes that were composed of bifunctional siloxane units having monovalent organic groups containing haloalkyl groups or epoxy groups and trifunctional siloxane units (see Japanese Patent Application Early Disclosure No. 5-88413 (1993).
However, these coating agents are cured by reactions between the aminoalkyl groups and the monovalent organic groups containing haloalkyl groups or epoxy groups. For this reason, there are the problems that the cured films that are obtained are generally brittle and lacking in durability.
In particular, salts remain as by-products in the cured films that are obtained via the reaction between the organopolysiloxanes having aminoalkyl groups and the organopolysiloxanes having haloalkyl groups. Because of this, the mechanical properties of the cured film are poor and electrification properties vary greatly with changes in humidity.
The inventors conducted intensive research for the purpose of solving the aforementioned problems. As a result, they arrived at this invention.
Specifically, The objective of this invention is to provide a coating agent for carriers for electrostatic latent image development that is cured rapidly by heating and that forms a cured film having electrification properties that are relatively unaffected by humidity and to provide a carrier for electrostatic latent image development having electrification properties which are relatively unaffected by humidity.